Cenozoic Anoxia

Lapses in oceanic circulation
Global conveyer belt ceases to function
Oxygen depleted in cataclysmic fashion

Heavy erosion, volcanic outgassing
Ozone depletion leads to mass extinction

Obstruction, deprivation, stagnation
Ultraviolet radiation, atmospheric penetration

Consequential suffocation from deoxygenation
Comprehensive homosapien fossilization